I can't see the thumbnails in my project line. forgot the path to delete these files so they reappear.
You can set them to be anywhere so there's no single answer.
Open FCP and go to System Settings > Scratch Disks.
It will show you the path...
Mine are at the top level of a particular drive. If yours are in a subfolder somewhere it will also show the directory path with the usual / separators.
mine looks the same
once there what do I drag to trash & delete so the images reappear in my timeline?
Just delete the contents of the Thumbnail Cache Files folder then restart FCP.
Retrieving data ...